1. Field of the Invention
The present invention relates to a mechanism for winding up and off an air-tight sheet on a transparent top plate of a contact printer.
2. Description of the Prior Art
Air-tight sheet winding mechanisms are designed to wind up an air-tight sheet of a flexible material such as rubber, and alternatively wind off and spread out the sheet on a transparent top plate of a contact printer. On the transparent plate are mounted an original and a photosensitive film. Examples of such mechanisms are shown in JAPANESE PATENT PUBLICATION GAZETTE No. 63-18182 and No. 61-42249, which generally have a structure described below.
An air-tight sheet has one end fixed to the front of the body of a contact printer, and the other connected to a spring roller. The spring roller winds up the air-tight sheet by rotating about an axis due to the elasticity of an internal coil spring thereof. A pair of linear track rails are installed on respective sides of the printer body between the front and the rear ends of the contact printer. A carriage engaging with each track rail is driven to move along the track rail by a driving system including an endless chain and sprockets. A roller support for supporting the spring roller on both ends in a rotatable manner are secured in between the carriages. The roller support is laid parallel to and over the transparent top plate.
The roller support is first drawn by the carriages towards the rear end of the printer, or in a direction against the elasticity of the internal coil spring of the spring roller. Accordingly, the air-tight sheet wound around the roller is spread out under a certain tension over an original and a photosensitive film on the transparent top plate. When the roller support is moved towards the front end of the printer, the spring roller rotates due to the elasticity of the coil spring and thereby the air-tight sheet is wound around the spring roller.
In spreading the air-tight sheet, the original and the photosensitive film are tightly brought into contact with each other by removing the air in between the top plate and the air-tight sheet spreading over the original and the photosensitive film. The original and the photosensitive film are then irradiated with light from a light source, so that the photosensitive film is exposed to make a negative film.
The original and the photosensitive film are fixed on the top plate by common pin bars. When a plurality of negative films are printed from a single original, the plurality of negative films sometimes show positional mismatch of characters or figures between each other. Namely, the photosensitive films are not accurately positioned against the original on the top plate but are shifted by some amount.
Such positional shifting may primarily be attributed to a shift of the photosensitive film, which is directly in contact with the air-tight sheet, from the original when the air-tight sheet is laid over the original and the photosensitive film on the top plate. The positional shift of the photosensitive film is caused, for example, by crumples of the air-tight sheet spread over the photosensitive film or a misalignment of both sides of the air-tight sheet wound around the spring roller.
In the conventional air-tight sheet winding mechanisms described above, high accuracy parts including the air-tight sheet are required to prevent the positional shift of the photosensitive film.
The conventional mechanism has independent driving systems disposed on respective sides of the contact printer. Two ends of the roller support for supporting the spring roller are independently drawn by the respective driving systems, and thus the roller support may move obliquely or unevenly against the moving direction.
The air-tight sheet winding mechanism requires fine adjustments about the following items on assembly to ensure its high precision: straightness and parallelism of the track rails disposed on both ends of the contact printer, tension of the endless chain of each driving system, synchronous actuation of the driving systems, and tension of the air-tight sheet.
While metal parts such as sprockets attain the high precision rather easily, the rubber air-tight sheet is not readily manufactured with similar precision in shape, thickness, and width to the metal parts. Heterogeneous physical properties of the rubber air-tight sheet cause waviness, residual stress, residual strain, and other unevenness during manufacture of the sheet.
Even when metal parts are manufactured with high precision and various fine adjustments are implemented on assembly and installment of the printer, such heterogeneous physical properties of the sheet may cause non-straight or oblique movement or waviness of the sheet during repeated winding-up and -off of the air-tight sheet; this results in positional shifting of the photosensitive film. Positional shifting should accordingly be checked and adjusted after every predetermined number of winding-up and -off operations of the air-tight sheet.
Namely, in the conventional air-tight sheet winding mechanism, the troublesome fine adjustments are often required to prevent positional shifting during continual operation of the contact printer.